This chapter will introduce some of the numerous devices that may be used to control the flight of a guided missile. We will discuss four types of control systems: pneumatic, pneumatic-electric, hydraulic -electric, and electric. Throughout the chapter we will deal with general principles, rather than the actual design of any specific missile.

5A2. Definitions

A missile GUIDANCE system keeps the missile on the proper flight path from launcher to target, in accordance with signals received from control points, from the target, or from other sources of information. The missile CONTROL system keeps the missile in the proper flight attitude. For example, the missile axis must lie along the desired trajectory, rather than at an angle. The missile must be roll stabilized; that is, a fixed plane through the missile axis must remain parallel to a fixed reference plane outside the missile. Flight attitude stabilization is absolutely necessary if the missile is to respond properly to guidance signals. For example, assume that the missile has rolled 90 degrees clockwise from the proper attitude. Now, if it receives a "right turn" command from the guidance system, operation of the control surfaces will actually turn the missile downward, rather than to the right. But if the control system keeps the missile in the proper attitude, guidance signals will be correctly interpreted, and will produce the desired correction in the missile flight path.

When the control system determines that a change in missile attitude is necessary, it makes use of certain controllers and actuators to move the missile control surfaces. The guidance system, when it determines that a change in missile course is necessary, uses these same devices to move the control surface. Thus the guidance and control systems overlap. For convenience, we will assume that the controllers and actuators are a part of the control system, rather than the guidance

To summarize: the missile control system, discussed in this chapter, is responsible for missile attitude control. The guidance system, discussed in chapter 6, is responsible for missile flight path control.

5A3. Purpose and function: basic requirements

The control system is made up of several sections that are designed to perform, insofar as possible, the functions of a human pilot. To accomplish this purpose, the control surfaces must function at the proper time and in the correct sequence. For example, in driving your car, you remember that you must make a turn at a certain distance from the starting point. You therefore anticipate the turn. In a missile control system, the remembering is done by INTEGRATING DEVICES and the anticipation is done by RATE DEVICES. These devices will be described later.

The first requirement of a control system is a means of a sensing when control operations are needed. The system must then determine what controls must be operated, and in what way. For example, the system may sense that the missile nose is pointing to left of the desired course. Obviously, right rudder is required. (Other missiles may make use of different control surfaces.) The length of time rudder control is needed depends on the size of the error. Should the attitude deviation be to one side and also either up or down, simultaneous action by rudder and elevator controls would be needed.

5A4. Factors controlled

Missile course stability is made possible by devices which control the movement of the missile about its three axes. The three flight control axes are shown in figure 5A1. These are the pitch, yaw and roll axes.

YAW. Missile movement about the yaw axis is controlled by the rudder. Other methods for controlling yaw will be covered in the following section of this chapter.

ROLL. Roll deviations are controlled by differential movements of rudders, elevons, or other flight control surfaces.

5A5. Methods of control

CONTROL SURFACES. The primary control surfaces of aircraft, and of some missiles, are rudder, aileron, and elevator. The functions of these surfaces are shown in figure 5A2. The top view shows how the rudder controls the direction of travel. The rudder is attached to a section of the tail structure called the vertical stabilizer. In addition to course control, the

The center view of figure 5A2 shows how the ailerons can control roll. The ailerons are attached to the trailing edges of the main lifting surfaces. When one aileron is lowered, the opposite aileron is raised. Usually, the ailerons are coupled to other surfaces in such a manner that good roll control is obtained.

The elevators are attached to a section of the tail assembly called the horizontal stabilizer. The elevators give pitch control; both elevators go up and down simultaneously.

A study of the drawings will show that control action is obtained by the control surfaces when they present opposition to air flow in such a manner that a force is produced. This force, pushing against the control surface, causes the wing or tail to which the surface is attached to move in a direction opposite to the control surface movement.

But this type of control is not suitable for use at high altitudes, because the air is so thin that it produces very little force against the control surfaces. High speeds introduce other problems so that the basic control surfaces

JET VANES. As explained in chapter 2, jet vanes maybe used to control the path of a missile. Figure 5A3 shows how a movable vane is installed directly in the jet exhaust path. When the position of the vane is changed, it deflects the exhaust and causes the engine thrust to be directed at an angle to the missile axis. Because of the tremendous heat built up by the burning fuel, the life of a control vane is short.

MOVABLE JETS. Figure 5A4 shows a gimbaled engine mounting. The engine is mounted so that its exhaust end is free to move and thus direct the exhaust gases in a desired direction. There are two serious objections to this method of control. All fuel lines must be flexible, and the control system that moves the engine must furnish considerable power.

The gimbaled engine mounting does not give full control about all three axes. It cannot control roll. To get control on all axes, two gimbal-mounted jets can be positioned as shown in figure 5A5. Both jets must be free to move in any direction, and each jet must respond to signals from any of the three control channels (pitch, roll, and yaw).

A control system using four jets is shown in figure 5A6. Each jet turns in only one plane. Two of the jets, Nos. 1 and 3, control yaw. Jets 2 and 4 control pitch, and all four jets are used together to control roll.

Positions of the jets are controlled by hydraulic cylinders linked to the engine housing. One cylinder and linkage is required for each engine. The direction in which hydraulic pressure is applied is determined by an electric actuator.

FIXED STEERING JETS. Figure 5A7 shows a fixed jet steering system. The jets are placed around the missile so as to give directional control by exerting a force in one direction or another. A missile using this control

Figure 5A3.-Jet vane control

Figure 5A4.-Jet control of direction.

Figure 5A7.-Fixed jet steering.

system has a smooth outside surface, since control surfaces are eliminated.

5A6. Types of control action

The basic control signals may come from inside the missile, from an outside source, or both. To coordinate the signals, computers are used to mix, integrate, and rate the signal impulses. We will first briefly discuss individual computer section functions so that you may see the part the computer plays in a complete control system. A more detailed discussion of computers appears later in this chapter.

MIXERS. The mixer combines guidance and control signals in the correct proportion, sense, and amplitude. In other words, a correction signal must have the correct proportion to the error, must sense the direction of error, then apply corrections in the proper amplitude.

PROPORTIONAL. The proportional control operates the load by producing an error signal proportional to the amount of deviation from the control signal produced by a sensor.

RATE. Rate control operates the load by producing an error signal proportional to the speed at which the deviation is changing. This output is usually combined with a proportional signal to produce the desired change in missile attitude or direction.

5A7. Types of control systems

We have previously discussed four aerodynamic control methods. These were control surfaces, jet vanes, movable jets, and fixed steering jets. There are four basic methods of moving these devices.

PNEUMATIC. A pneumatic control system is shown in figure 5A8. (Some of the operating controls shown are not directly a part of the

Keep in mind that the rate signal is proportional to the speed at which a missile deviation is changing in magnitude. This change is different from the displacement signal, which is proportional to the missile angular deviation at any instant. At the azimuth rate gyro, the rate signal appears as an unbalanced air pressure between two holes in an airblock pickoff.

Now, let us see what happens when the missile yaws and its nose veers to the right. A displacement gyro signal develops at the pickoff (azimuth control air jet). The jet then pivots to increase air pressure in the left hole of the pickoff. The air pressure is fed through the lower of the two air tubes to the diaphragm of the air relay. This forces the diaphragm to the left which admits high-pressure air that, in turn, forces the azimuth servo motor to the right. This motion is then transferred through a mechanical linkage to the rudder, which moves to the left and corrects the nose-right deviation.

Another stabilizing action takes place as the missile nose veers off course to the right. A signal is produced by the azimuth rate gyro as the nose moves. The azimuth rate gyro exerts a force on the right restraining spring, because the force on the gimbal precesses the gyro. As it precesses, more air is received by the left hole of the azimuth rate pick off. This increases the pressure in the same tube that contains the high pressure signal from the displacement gyro. The rate gyro is supporting the correction being exerted by the displacement gyro.

As the missile path deviates from its desired heading, the rate signal increases the corrective action of the displacement gyro. Therefore, if the deviation from proper heading were increasing at a rapid rate, the corrective signal from the rate gyro would be large and would quickly reduce the deviation of the missile.

At the instant when the missile has veered as far to the right as it is going to go, the error signal from the displacement gyro is greatest because the error is greatest. However, there is no signal from the rate gyro because the missile is not changing its heading at that instant. The rate gyro has been returned to its mid-position by the restraining springs.

A study of this drawing will show the advantages of having a counteraction between the rate signal and the displacement signal. If the two signals were in the same direction, the rudder would be farther away from the center axis of the missile and the missile would be heading back to the desired course at a faster rate. However, the rate of return would be so rapid that the missile would swing past the correct heading in the opposite direction. The missile would then be veering off course to the left so that the control system would need correction signals for that direction.

The cross-course variations would continue, with the missile wobbling back and forth on both sides of the desired heading. This action

The same kind of action is used in the displacement and rate controls in the pitch channel. The fundamental rate gyro or rate circuit output is the same for all missile flight surface control systems.

PNEUMATIC-ELECTRIC. The pneumatic control system we have just described can be combined with other systems to refine the control action. Electric signal pickoff are accurate and dependable. They can provide a signal voltage that is proportional to displacement. They have a decided advantage over pneumatic systems in transporting information over wires, instead of through tubing.

It is difficult to design a small electric motor with sufficient speed and power to actuate missile flight control surfaces. But we can combine the best features of electric and pneumatic systems as in figure 5A10. Electrical equipment is used in the front end, and operates pneumatic servos at the actuating end. A system like that in figure 5A10 is suitable for controlling a small, subsonic, short-range missile.

Pneumatic controls are slow because air is compressible, and time is required to build up enough pressure in a cylinder to move the piston. Since the piston is linked mechanically to the flight control surfaces, there is a time

The increase in response speed is obtained by allowing air to escape, through ports, into a relief valve after the servo valve has moved a certain distance from midposition. The relief valve lets high-pressure air into the boost cylinder, which then acts in parallel with the actuator cylinder to move the flight control surfaces. The additional force provided by the boost cylinder makes it possible to obtain large control surface deflections in either direction.

The sensors for a pneumatic-electric system are electric pickoffs that detect gyro displacement and produce a voltage proportional to the heading deviation angle. This voltage is small, and must be amplified before it can operate a solenoid and air servo valve.

The change from electric to pneumatic operation takes place at the air servo valve. The air servo motor rotates the torque tubes which are connected to the control surfaces and extend into the center section of the missile. The system shown in figure 5A10 may be used for either pitch or azimuth control.

HYDRAULIC-ELECTRIC. This combination is similar to the pneumatic-electric, except that the actuators are moved by hydraulic fluid pressure instead of air pressure. This removes some of the disadvantages of a pneumatic system, since the fluid is not compressible.

In a hydraulic-electric system, a continuously operated pump maintains hydraulic pressure during the flight. The hydraulic fluid is circulated in a closed system, so that it can be used over and over. Thus the operating time of the hydraulic components is unlimited, and the system is suitable for long range missiles.

Variations in pitch, roll, and yaw are sensed by gyro reference units with electric pickoffs. The pickoff voltages are fed to amplifiers and computers and are then used to operate a controller. The controller is usually a hydraulic

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Figure 5A11 shows a hydraulic-electric system for roll control. (In some missiles the ailerons are replaced by control devices called "rollerons".) The system uses proportional control only, which means that the controls react to information that shows the deviation of the missile axis from the desired flight path. The displacement signal is proportional to the deviation.

Should roll develop, the gyro (fig. 5A11) will detect it and cause the synchro to produce an error signal. The correction signal to the servo amplifier is the difference between the followup signal and the gyro signal, as indicated by the minus sign in the circle between the synchro block and the servo amplifier triangle.

The difference signal is amplified and used to operate the controller, which is a hydraulic transfer valve that regulates the flow of fluid to the cylinder. The piston in this cylinder operates the ailerons (rollerons; controllable jets; etc.) through mechanical linkages.

The equipment represented by the block labeled "jitter" provides an a-c voltage with a frequency of about 25 cycles per second. This is applied to the transfer valve and other equipment, to keep them in constant vibration and prevent the friction that may develop when the parts are not moving.

Variations in roll, pitch, and yaw are sensed by electrical components such as synchros or reluctance pickoffs. The signals from these components are fed to a computer which determines the amount of error, if any. Rate signals are obtained from the electrically driven rate gyro, or from an electric rate circuit operated by a displacement gyro signal. The control voltages are amplified and fed to controllers, which vary the power supplied to the motors that operate the flight control surfaces. A small motor running at high speed has the same power capability as a large motor running at low speed. Therefore a small, high-speed motor can be connected to the control surfaces through a reduction gear to secure the necessary torque.

A constant-speed motor, operating through a clutch, is best suited for rapid control operation, because the gear train inertia tends to cause an undesirable lag in control surface response. The lag may be great enough to make the missile oscillate about the desired trajectory. The use of a clutch helps to overcome this effect.

The speed of the controller drive motor must remain reasonably constant, regardless of loading. Otherwise, if the pitch output decreases, the speed of the motor would decrease; this would result in decreased output from the roll and yaw generators at the same time. As a result, there would be undesirable cross-coupling between control channels so that the pitch signal would affect other channels, and vice versa.

5A8. Energy sources

The energy required to operate the control surfaces may be taken from any of the following sources.

MISSILE ENGINE. The propellant of the missile may be used as a source of energy to operate the control system. Figure 5A13 shows how a generator, a hydraulic pump, or

Figure 5A13.-How energy is obtained from the missile engine.

Figure 5A14.-Auxiliary engine power.

This system does not take power from the main engine and would be suitable for use when the main propulsion unit was cut off. The auxiliary engine will furnish power so long as fuel is available. If necessary, the auxiliary engine could be used to drive a generator and pump simultaneously to obtain power for hydraulic-electric or penumatic-electric systems.

BATTERIES. Storage batteries can be used either by themselves or to drive motor-generator sets. The generator can supply

COMPRESSED GASES. Air and other gases can be stored in tanks under pressure for use in operating control systems. As explained in the section on pneumatic controls, the compressed gas is exhausted during the control operation and cannot be used again.

Turbine generators can be driven by the gases given off by BURNING FUEL CARTRIDGES if power is needed for a short time only. Such a power system is suitable for air-to-air missiles.

Missile control is similar to any automatic control function. The system corrects some controllable quantity, and then checks the results as a basis for further corrections.

There are four requirements of any automatic control system. Obviously, the first is something to control. The second is a means of determining when any controllable item has departed from a desired condition. The third is a means of converting an error signal into a form that can be used to regulate the controlling device. The last is the device that performs the actual control operation.

Factors that must be controlled by the missile control system are pitch, roll, and yaw. The system must provide a means of determining when the missile has departed from the desired attitude. Deviations are sensed by gyros. Electrical, mechanical, and electronic components are interconnected to form a complete control system.

The control system shown in figure 5B1 can set up fixed reference lines in space, from which deviations in attitude can be measured. It provides a mechanical and electrical means for operating the missile flight control

5B3. Error-sensing devices

Deviations in missile pitch, roll, and yaw are detected by gyros. A minimum of two gyros is necessary for missile flight stabilization. Each gyro sets up a fixed reference line from which deviations are measured. One such reference is the spin axis of a vertical gyroscope; from this axis deviations about the pitch and roll axes can be measured, as shown in figure 5B2.

A second reference line is the spin axis of a horizontal gyro, set up parallel to the horizontal axis of the missile as shown in figure 5B3.

Gyros used for missile control applications are divided into two classes: gyros used for

Figure 5B2.-Vertical reference line.

Figure 5B3.-Horizontal reference line.

stabilizing (control) purposes and gyros used for both guidance and stabilization. If turns or other maneuvers are necessary, a third gyro is required so that there will be one gyro for each sensing axis.

In addition to the control signals from the vertical and horizontal gyros, which are proportional to the deviation of the missile from the desired trajectory, a signal that is proportional to the rate of deviation is required for accurate control and smooth operation. A RATE GYRO furnishes the rate of deviation signal.

A gyro that is being used for rate deviation indications has a restricted gimbal that is free to rotate about one axis only. The spin axis of a yaw rate gyro is mounted parallel to the missile line of flight. The roll rate gyro spin axis is parallel to the missile pitch axis, and at right angles to the line of flight. The pitch rate gyro spin axis is parallel to the yaw axis of the missile and at right angles to the line of flight.

Figure 5B4 shows, in block form, a system used to sense motion of a missile with respect to one control axis. This system uses both a rate gyro and a free gyro. The gyro output signals are fed to an amplifier, which adds them and gives an output voltage proportional to their sum. This voltage is applied to a servo motor, which positions the flight control surface so as to drive the error amplitude and the rate of change toward zero.

5B4. References

In order to accurately determine errors, the complete control system must have reference values built in. The system is then capable of sensing a change, comparing the change to a reference, determining the difference, then starting a process that will reduce the difference, to zero.

The reference units in a missile control system are of three kinds-voltage references, time references, and physical references. A more detailed discussion of references will appear in the next section of this chapter.

5B5. Correction-computing devices

We have shown that sensor units detect errors in pitch, roll, and yaw, and that a reference unit furnishes a signal for comparison with the sensor output.

5B6. Power output devices

The amplification of error signals is performed by a conventional vacuum-tube amplifier or a magnetic amplifier. Regardless of the method used, the prime purpose of an amplifier is to build up a small sensor signal to a value great enough to operate the controls.

5B7. Feedback loops

For smooth operation of the controls, it is necessary to feed back some of the control power so that it counteracts some of the original force. There are two feedback paths. The major path represents information on the angular movement of the missile. This information is fed back to the sensor. The second feedback path, called the minor path, returns information on the reaction of a control surface, rather than the reaction of the missile. The use of feedback prevents control surfaces from swinging to the limit of motion, and thus avoids overshooting as the missile returns to the proper attitude.

The reference device provides a signal for comparison with a sensor signal, so that

5C2. Types of reference

In the following discussion, the three types of reference signals will be described separately to show how each type functions in the complete control system.

VOLTAGE. In some control systems, the ERROR SIGNALS are in the form of an a-c voltage which contains the two characteristics necessary to make proper corrections in the flight path. These are the amount of deviation, and the direction of sense of the deviation.

The amount of deviation maybe indicated by the amplitude of the error signal so that, as the deviation increases, the amplitude increases; and if the deviation decreases, the amplitude decreases. Therefore, when the missile attitude has been corrected and there is no longer a deviation, the error signal amplitude drops to zero.

The direction of deviation may be carried by the a-c signal as a phase difference with respect to the phase of a reference signal. Only two phases are required to show direction of deviation about any one control axis. When a phase-sensitive circuit, such as a discriminator, is used to compare the error signal with the a-c reference signal, the direction of error is established and the output containing this information is fed to other control sections.

In most cases, the a-c reference voltage is the a-c power supply for the control system. It also furnishes the excitation voltage for the sensor unit that originates the error signal.

TIME. The use of time as a reference is familiar to everyone. One common application is in the automatic home washer. A clock-type motor drives a shaft, which turns discs that operate electric contacts. These contacts close control circuits that operate hot- and cold-water valves, start and stop the water pump, change the washer speed, spin the clothes dry, and finally shut off the power. Each operation runs for a specified time interval. This kind of timer can be used for certain missile control operations.

Timer control units vary considerably in physical characteristics and operation. All of them require an initial, or triggering, pulse. Since all timers in a complete system are not triggered at the same time, each must have its own trigger. This is usually an electrical signal. It may be fed to a solenoid which mechanically triggers the timing device.

Another triggering method involves the application of an electrical signal to a heater coil which heats a bimetal strip and causes it to bend, thus opening or closing electrical contacts. This method maybe more familiar when you contemplate the operation of a typical thermostat like the one found in the home. Still another triggering method is to apply an

Mechanical timers are used in some missile control systems. In operation, these timers are similar to mechanical alarm clocks. The energy is stored in a main spring. If a mechanical timer is used in a missile, the clock mechanism is not started until the missile is in flight, and therefore some form of triggering linkage is necessary. This usually consists of a catch that can be released by a solenoid. Since the spring cannot be rewound after the missile has been launched, a mechanical timer can be used only once during a flight.

Electrical timers in missile control systems are divided into motor types and thermal types. The triggering of either type is done by

An arm connected to the output shaft serves as part of a switch contact system. The length of time required for the arm to travel from the starting position to the point where contact is made is the delay time of the unit. Normally, this mechanism is used only once during a missile flight. If recycling is necessary, a more complex unit is required.

Thermal delay tubes and relays may also be used to control time delay actions. Thermal delay devices have the advantage over clock timers in that they can be made to recycle

One type of thermal delay tube is shown in figure 5C3. Its components are the two bimetal strips, the contacts, a heating coil, and the strip supports. When a triggering voltage is applied, the heating coil heats ONE of the bimetallic strips. As the temperature rises, the strip deforms and its contact moves toward the other contact. When the bimetal strip has heated sufficiently, the contacts touch and the output circuit is completed.

The amount of time between application of the triggering voltage and closing of the contacts is determined by the contact spacing, the temperature characteristics of the metals in the strips, and the characteristics of the heater coil. The delay time is preset by the manufacturer; the assembly is then placed in a tube-type enclosure, and the air is pumped out of the tube. This type of construction prevents any adjustment of the time delay.

The effect of ambient temperature variations can be avoided by making both strips of the same metals. Then, when one strip deforms

PNEUMATIC. Pneumatic timers may be used in certain missile applications. Time delay action is obtained by compressing air in a cylinder and then allowing the air to escape through a small orifice.

There are two general types of pneumatic timers-piston and diaphragm. The piston type is shown in figure 5C4. The felt washer acts as an air seal. As the plunger is pulled up, the spring is compressed and the contacts are opened. The spring is held in compression by the inertia block.

The inertia block forms the trigger for the timer. The block is of metal and will be thrown backward when it is subjected to sufficient accelerations. Thus, when a missile is launched, the block will fly back and release the catch that holds the spring under compression. Spring pressure will then push the piston downward, forcing air out through the orifice. The orifice opening may be changed by adjusting the needle valve. The smaller the orifice,

The diaphram-type pneumatic timer, shown in figure 5C5, operates on essentially the same principle as the piston-type timer that has been described.

Most missile control systems use some form of timer. Remember that an individual timer may be used to start a variety of control functions. Sometimes a timer is used strictly as a safety device.

PHYSICAL REFERENCES. There are a number of references for missile control systems other than the voltage and time classifications we have discussed. The remaining types have been grouped under the heading of physical references. They include gyros, pendulums, magnetic devices, and the missile airframes.

GYROS. We have already explained how a gyroscope establishes a reference line in space. A gyro pickoff system can sense any change in missile attitude with respect to that reference.

Some gyros are precessed to a vertical position by a pendulum device called a "pendulous pick-off and erection system." The complete gyro system is called a vertical gyro; it maybe used to measure the pitch and roll of a missile.

MAGNETIC DEVICES. Magnetic compasses have been used for centuries to navigate the seas. The compass enables a navigator to use the lines of flux of the earth's magnetic field as a reference. A similar device, known as a "flux valve" is used in some missile control systems. Its primary purpose is to keep a

MISSILE AIRFRAME. The airframe of the missile must be used for certain references. For example, the movement of flight control surfaces cannot be referenced to the vertical, or to a given heading, because such references change as the missile axes change. Therefore,

Synchro indicators can be used to indicate the angular position of control surface with respect to the missile airframe. A potentiometer can be used in the same way by mounting in on the missile airframe so that its shaft will be driven by the flight control surface movements.

The sensor unit in a guided missile control system is a device used to detect deviation from the desired attitude. In this section, we will discuss the use of gyroscopes, altimeters, and transducers as sensing units. Gyroscopes are generally considered to be the basic sensor unit in any missile control system. Other types of sensors, such as altimeters and transducers, are classed as secondary units.

5D2. Gyros

A gyroscope contains an accurately balanced rotor that spins on a central axis. Figure 5D1 shows a FREE GYRO that is mounted so it can tilt, or turn, in any direction about its center of gravity.

GYROSCOPIC INERTIA. The characteristic of a gyroscope that resists any force which tends to displace the rotor from its plane of rotation is called "gyroscopic inertia." Three factors determine the amount of inertia. These are: the weight of the rotor, the distribution of this weight, and the speed at which the rotor spins.

A gyro with a heavy rotor has more rigidity than one with a light rotor, if the speed of rotation is the same for both. Distributing the weight of the gyro to the outer rim of the rotor will give increased rigidity even though there is no increase in the weight of the rotor. An increase in gyro rigidity can also be obtained by increasing the speed of rotation.

PRECESSION, REAL AND APPARENT. The characteristic of a gyro that causes the rotor to be displaced in a direction 90 degrees from that of the applied force is called precession. There are two types of gyro precession: REAL and APPARENT. Real

Figure 5D1.-Free gyroscope.

precession is sometimes called INDUCED PRECESSION.

The direction in which a gyro will precess, when an external force is applied, is shown in figure 5D2. A force applied to a gyro at its center of gravity does not tend to tilt the spin axis from its established position, and therefore does not cause precession. A spinning gyro can be moved in any direction without precession, if its axis can remain parallel to its original position in space. Therefore, the gyro can measure only those movements of the missile that tend to tilt or turn the gyro axis. Two gyros are needed for vertical and horizontal stabilization of missile flight. The spin axes of these gyros would be at right angles to each other.

APPARENT PRECESSION. The axis of a spinning gyro points in a fixed direction

Figure 5D2.-Gyro precession.

because inertia fixes it in space. Over a period of time a gyro axis will appear to tilt. This is called apparent precession, and is due to the rotation of the earth.

Figure 5D3 represents a gyro at the equator, with the spin axis horizontal and pointed east-west. The earth turns in the direction shown by the arrow. If you could observe the gyro spin axis from a point out in space, it would appear to always point east. To an observer standing on earth, the spin axis appears to gradually tilt or drift, so that after three hours, the spin axis has tilted 45

Figure 5D3.-Gyro position in space about the earth.

This action gives the impression that the gyro has turned end for end, and that a complete revolution is made every 24 hours. But this is not true. Actually, the gyro axis has maintained its fixed direction in space; only the earth has moved.

The apparent precession of a gyro makes it unfit for use as a reference over an extended time unless some kind of compensation is used to keep the gyro in a fixed relation to the earth's surface.

GYRO DRIFT. Gyro error caused by random inaccuracies in the system is called drift. It has three principal causes-unbalance, bearing friction, and gimbal inertia.

Dynamic unbalance may occur because of operation at some speed or temperature other than for which the gyro was designed. Some unbalance exists in any gyro because of manufacturing tolerances.

An even amount of bearing friction all around a shaft does not cause drift. It will, however, cause the speed of rotation to change.

Figure 5D4.-Apparent precession.

Energy is lost whenever a gimbal rotates, because of inertia. The larger the mass of the gimbal, the greater the drift from this source.

MOUNTING SYSTEMS. The main cause of random drift in gyros is friction in gimbal bearings. Figure 5D5 shows one type of mounting that has been developed to reduce this friction. It is called FLOATED GYRO UNIT.

The floated gyro unit is a viscous-damped, single-degree-of-freedom gyro with a microsyn torque generator and a microsyn signal generator mounted on its output shaft. The microsyn torque generator places a torque on the gyro gimbal.

The term "single-degree-of-freedom" means the gimbal is free to rotate with respect to the gyro case about a single axis. This axis is called the output axis; it is perpendicular to the gyro spin (reference) axis. If an angular velocity acts on the gyro case with a

In figure 5D5 the gyro wheel is contained within the damper housing. The microsyn signal generator units are mounted on the gyro shaft as shown in the drawing. The space between the damper housing and the gyro case is filled with a viscous damping fluid. Because of the high specific gravity of the fluid, it serves to float the gyro damper housing and gyro gimbal shaft, and thus reduces the gimbal bearing friction and drift. Thermostatically controlled heaters around the damping fluid space keep the fluid viscosity constant. If the gyro shown in figure 5D5 were mounted with its input axis parallel to the pitch of yaw axis of the missile, the torque applied to the output shaft would be proportional to the difference between the desired angular velocity of the missile and its angular velocity about the axis.

Another form of gyro support, shown in figure 5D6, is known as an air bearing. This type of support reduces friction to such a low value that for all practical purposes it can be

ERECTING SYSTEMS. Figure 5D7 is a block diagram that shows how signals from a precession sensor are used to maintain gyro stability. If the gyro spin axis in this system is vertical, the gyro output signal will be zero. If the spin axis moves away from the vertical, the gyro will send a voltage to the precession sensor. The amplitude of this voltage will depend on the amount of precession, and its phase of the direction of precession. The precession sensor output is amplified to operate the torque motor, which returns the gyro to the vertical position.

Horizontal gyros use a leveling system to keep the spin axis horizontal, and a slaving system to stabilize its direction. The slaving system uses a fluxvalve to sense the direction of the earth's magnetic field. The flux valve

The gyro's spin axis, kept tangent to the earth's surface and slaved to the earth's magnetic field, provides a basic reference for missile heading. When the missile turns in either direction, the flux valve turns with it. When the flux valve turns, the angle between its coils and the earth's magnetic field is changed, and an error signal is generated. The error signal is amplified and used to operate controls that correct the missile heading.

RATE GYROS. The control signals furnished by the vertical and horizontal (free) gyros are proportional to the deviation of the missile. Another signal, proportional to the RATE of deviation, is required for smooth control. This is the RATE-OF-DEVIATION signal; it is supplied by a RATE gyro. A rate gyro has a restricted gimbal that is free to rotate about only one axis. Its construction is shown in figure 5D8.

A YAW RATE GYRO is mounted with its spin axis parallel to the missile line of flight.

Displacement signals alone would give the missile a tendency to over-correct its errors, and yaw or pitch about its desired course. The displacement and rate of change signals minimize over-correction, and ensure stability.

PICKOFF SYSTEMS. A "pickoff" is a device that produces a useful signal from the intelligence developed by a sensor. The sensing devices for missile control generally indicate angular or linear displacement, measured with respect to some fixed quantity. The pickoff must be able to measure the amplitude and direction of the sensor displacement, and produce a signal that represents both quantities. Electrical pickoffs use phase relation or polarity difference to indicate direction. The ideal pickoff should have a linear output and minimum friction loss.

5D3. Altimeters

An altimeter measures altitude. There are two main types: PRESSURE and ABSOLUTE.

The PRESSURE type operates on the principle that air (atmospheric) pressure is

A pressure altimeter is a form of aneroid barometer. Its mechanism includes a bellow s-like chamber from which most of the air has been removed. The pressure of the atmosphere tends to collapse the bellows. The surface of the bellows is connected to a scale pointer through a mechanical linkage, which magnifies the bellows surface movement.

As the pressure does not remain constant at any one level, this type of altimeter may have an error due to variable atmospheric conditions.

The ABSOLUTE altimeter is sometimes called a radio altimeter. It indicates altitude above the ground, rather than above sea level. It is actually a form of radar, since it measures the time required for a radio pulse to reach the ground, be reflected, and return.

The transmitter antenna sends an FM signal straight down. The reflected energy is picked up by a separate antenna. A detector combines the reflected signal with a sample of the transmitted signal and generates a difference frequency. This frequency is determined by the height above the ground. The detector output is amplified and fed to an indicator such

This system accurately indicates height above the ground, but it can not indicate height above sea level.

5D4. Air-speed transducers

A guided missile may use a transducer to measure ram air pressure, and provide an output voltage that indicates missile air speed.

Another type of airspeed transducer uses a synchro generator. The rotor of the synchro is so connected that expansion or contraction of the bellows causes the rotor to turn. This type of sensor will be described in the following section.

The pickoff device is important to the missile control system because it produces a signal from the intelligence developed by a sensor unit.

5E2. Requirements

The signal produced by the pickoff must be suitable for use in the control system it is serving. The pickoff must have an output sense. That is, it must be able to determine the direction of displacement and then produce a signal that indicates the direction. In electrical systems the indication may be a phase or polarity difference.

The ideal pickoff should have a considerable change in output for a small movement of the pickoff. It should also have minimum torque or friction loss since these losses would be reflected to the sensor element and affect its operation. Small physical dimensions and light weight are additional requirements for pickoffs used in missiles. The null point (no output) should be sharply defined.

5E3. Type

Electrical pickoffs in common use fall into four categories. Each has some characteristic that makes it suitable for certain applications.

SYNCHRO PICKOFFS. A synchro pickoff device is normally composed of a pair of synchro units wired as a generator and synchro motor. When an exciter voltage is connected to the pair, movement of the generator rotor will produce a corresponding movement of the synchro motor rotor. The generator rotor

To get better action as the null point is approached, a differential synchro system is sometimes used. In this system, two inputs-one electrical and the other mechanical-are fed to a synchro differential generator unit, which then furnishes a voltage equal to the sum or difference between the two inputs.

Synchro pickoffs are sometimes called selsyns, autosyns, or microsyns.

POTENTIOMETERS. A potentiometer is a variable resistance that is normally used as a voltage divider. The resistance element is formed into a circular shape and a moving arm makes contact with the element. By connecting leads to the ends of the strip from a voltage supply and then connecting a load to the moving arm and one end of the strip, the source voltage may be divided by varying the position of the arm on the strip.

The resistance used for many electronic applications is composed of a thin film of carbon deposited on an insulating material. This type of resistance element is not suitable for servo applications because the resistance changes with temperature, humidity and wear. These disadvantages are overcome by using a wire-wound resistance strip, as shown in figure 5E1.

Figure 5E2 shows how a potentiometer divides voltage. The source voltage is applied

Figure 5E1.-Wire-wound potentiometer.

to points A and B. One side of the load is also connected to B. The section of the resistance strip between the moving arm and A acts as a resistance in series with the load, and there is less voltage at the load that is being furnished by the source. If the moving arm is all the way down to B, the load will get no voltage.

Thus, the position of the moving arm determines the amount of voltage. It is also possible to use the variation in resistance as a control medium. Since the resistance between A and the moving arm and between B and the moving arm vary as the arm is moved, a null can be indicated when the two resistances are equal.

If the shaft of the potentiometer is mechanically coupled to the sensor, the output voltage will vary according to the moving arm displacement. However, the voltage does not change smoothly with this type construction. The jumpy output is due to the voltage

Figure 5E2.-How a potentiometer divides voltage.

Potentiometers are often used in bridge circuits. The potentiometer forms two arms of the bridge, and fixed resistors form the other two. It is also possible to use two potentiometers to comprise all four arms of the bridge.

RELUCTANCE PICKOFF. A variable reluctance pickoff and gyro rotor are shown in figure 5E3. (The gyro rotor is the cylindrical object in the drawing.) The rotor is made of ferrous material, which has been slotted lengthwise and the slots filled with brass.

The pickoff element is an E-shaped metal mass, of which the center arm is a permanent Alnico (special alloy of aluminum, nickle, and cobalt) magnet. The outer arms, and the long side of the E, are made of soft iron. Coils are wound on each leg and connected in series opposition. As the gyro rotates, it causes regular variations in the magnetic flux paths as the brass and ferrous strips pass the end pieces. This establishes regular variations in flux density and induces an a-c voltage in the coils. However, because of the opposing connection of the coils, the induced voltages cancel.

The gyro spin and precession axes are shown by dashed lines. As the rotor precesses, the air gaps at each end vary oppositely (one increases and the other decreases) in proportion to the precessing force, and cause different induced voltages in the pickoff coils. Since the two voltages are different they no longer cancel, and the output voltage is that produced by acceleration. The output voltage is rectified by the crystal diode and filtered by the condenser. The resulting d-c voltage can be used as an acceleration signal. The polarity of the signal voltage depends upon which coil has the greatest induced voltage, and therefore indicates the direction of the acceleration.

Figure 5E4 shows another type of reluctance pickoff. The stator has four coils divided into two pairs. One pair is supplied with a constant-amplitude a-c voltage from a reference oscillator.

Voltage from one pair of coils is induced in the second pair through an armature. The

515354 O-59-7

CAPACITANCE PICKOFF. As shown in figure 5E5, a capacitance pickoff is composed of two outer plates that are fixed in position.

This change in capacity can be used to vary the tuning of an oscillator. The change in oscillator frequency is then used for sense control. This type of pickoff is the most sensitive of all, since a very slight change in plate spacing will cause a large change in frequency.

Computers appear in missile systems in a variety of forms. The computer maybe a simple mixing circuit in a missile, or it may be a large console type unit suitable for use at ground installations only.

5F2. Function and requirements

One important function of a computer is the coding and decoding of information relating to the missile trajectory. It is necessary to code and decode control information in order to offset enemy countermeasures and to permit control of more than one missile at the same time.

Another function of the computer is the mixing of signals from sensor and reference units to produce error signals. Figure 5C1 shows, in block form, how the computer is linked with other sections of the complete system. The signals from the sensor and reference units may be mixed in a preset ratio, or they may be mixed according to programmed instructions.

The error signals produced by mixing are amplified and passed to the control actuating system and the followup section. The output of the followup section is then fed back to the computer for reprocessing. The purpose of

The computer section may also compare two or more voltages to produce error signals. For this purpose, voltage or phase comparator circuits are added. The synchro units discussed in the previous section are used in computers to convert signal voltages into forms that are better suited for processing.

Airborne computers are generally classified according to the phase of missile flight in which they are used. The computers may be separate units or they may be combinations of prelaunch computer, launch computer, azimuth computer, elevation computer, program computer, and dive-angle computer.

5F3. Types of computers

In a missile control system, computer elements are of general types-mixers, integrators, and rate components.

MIXERS. As you will recall from the first part of this chapter, a mixer is basically a circuit or device that combines information from two or more sources. In order to function correctly, the mixer must combine the signals that are fed to it in the proper PROPORTION, SENSE, and AMPLITUDE.

Electronic mixers may use a vacuum tube as a mixing device. Probably the most common type of tube mixer is the one used in conventional superheterodyne radio sets. Here a tube mixes an incoming RF signal with the signal of a local oscillator to produce a difference frequency. It is also possible to use a network composed of inductors, capacitors, and resistors for mixing. Regardless of the type of mixer, the signals to be combined are represented by the amplitude and phase of the input voltages. Voltages from such sources as pickoffs, rate components, integrators, followup generators, and guidance sources may be combined by the mixer section to form control signals.

Mechanical mixers consisting of shafts, levers, and gears can also be used to combine information. Figure 5F1 shows how lateral signals from two sources can be combined by using plain levers. To see how this works, assume that shafts 1 and 2 operate independently, and that their positions represent information that must be combined. The three connections pivot freely. The position of shaft 3 represents a weighted average

Another mechanical mixer uses gears to combine position or angular velocity information. The gear arrangement is similar to that of an automobile rear axle differential. If the input shafts contain position information, they will move slowly and maintain approximately the same average position. The position of the output shaft constantly indicates the difference between the two shaft positions. If the information is represented by the speed of the shaft rotation, the angular velocity of the output shaft represents the difference between the two input shaft speeds.

It is possible to arrange the input shafts so that the output represents the sum of the inputs rather than the difference. Weighting factors can be controlled by changing the gear ratios in the differential.

Sometimes information is transferred through air or hydraulic tubes. The signals are created by varying the pressure inside the tube. Two signals can be combined by joining two tubes into one.

INTEGRATORS. An integrator performs a mathematical operation on an input signal. The

But, an actual missile error signal is not constant, as we assumed in the above example. The amplitude and sense of the error change continuously. Even so, the integrator output is proportional to the product of the operating time and the average error during that time. Should the sense of the error change during the integration period, a signal of opposite sense would cause the final output of the integrator to decrease. The integrator can be considered as a continuous computer, since it is always producing a voltage that is proportional to the product of the average input voltage and time. Therefore, the integration of an error with respect to time represents an accumulation of intervals of time and errors over a specified period.

Any integrator has a lag effect. To see why this is true, let us visualize a situation like that shown graphically in figure 5F2. The solid lines forming the rectangles represent on-off signals plotted with respect to time. The polarity is represented by the position of the rectangle above or below the time reference line. The heavy white lines represent the integrated output signal.

Although the input signal goes from zero to maximum with zero time lag, there is no output at that instant. The graph shows the lag effect; note that time is required before the output reaches an appreciable amplitude. Approximately the same length of time is required for the output amplitude to drop to zero after the input pulse ends.

The figure also shows the additive effect of two successive negative pulses. This action is made possible by the time lag, and is used to give more precise control action.

The output signal from the integrator is used to support the proportional error signal, to make sure that enough correction will always be made by the control system.

Keep in mind that the degree of control exerted by a pure proportional (unamplified) signal is limited. Over-control, or undercontrol, cause excessive movement of the missile about the desired trajectory. There are times when

Integration may be performed by a motor, the speed of which is proportional to the amplitude of the input signal. The motor drives a pickoff, and the distance the pickoff moves is proportional to the integral of the input signal.

The direction of motor rotation will depend on the polarity or phase of the input signal. The amplitude of the error signal varies irregularly; the sense of the signal may reverse, causing reversal of the motor rotation.

Other types of integrators use ball-and-disk mechanical arrangement, resistance-capacity (R-C) circuits, resistance-inductance (R-L) circuits, and thermal devices.

RATE SYSTEMS. The rate section in a missile control system should produce an output signal proportional to the RATE OF CHANGE of the input signal amplitude.

The preceding section showed that a time lag is present in integrator circuits. It is this time lag that makes rate circuits necessary. Missile deviation cannot be corrected instantly, because the control system must first detect an error before it can begin to operate.

The ideal control system would have zero time lag, thus permitting zero deviation during the missile flight. All design efforts are toward a control system with this degree of perfection. Control surfaces are designed to correct missile flight deviations rapidly. The control surfaces are moved rapidly by actuators, which are operated by amplified error signals. But it is possible to have a signal so large that the missile is driven beyond the desired attitude, and an error occurs in the opposite direction. This error drives the missile back in the first direction. The end result is a series of swings back and forth across the desired trajectory.

These unwanted swings are known as oscillation (or hunting) and the addition of a rate signal has the effect of damping (retarding)

The end effect of a rate signal is a reduction in the time between the initial control pulse and the output action. To reduce this time, the rate signal is combined with the proportional signal to produce a resultant signal that leads the original proportional signal.

However, there is output from the rate device only when the missile deviation is changing. The amount of output is dependent on the rate of change. By combining the rate signal and the error signal, the system can be made to respond to a constant error. It is also

Perhaps the most common method of producing a rate signal is by using a separate sensor unit, such as a rate gyro. As explained previously, the rate gyro construction is such that it can precess only a few degrees and in only one plane. Precession is restrained by a spring that tends to return the gyro to the midpoint. Any precession in this plane is caused by a force acting on the gyro gimbals. Such a force would be developed by any angular movement of the missileframe. The magnitude of the force would be proportional to the rate of movement. The gyro displacement is detected by a pickoff, and the output of the pickoff is the rate signal.

Amplifiers are divided into two groups-POWER and VOLTAGE. Both are used in missile control systems to build up a weak signal from a sensor so that it can be used to operate other sections of the control system. These sections normally require considerably more power or voltage than is available from the sensor. Most amplifiers use electronic tubes, but in this section we will discuss some of the less conventional amplifiers.

5G2. Operating principles

Some functions in missile control systems require a series of flat-topped pulses, called square waves, at a definite frequency. It is possible to convert other wave shapes to square waves with vacuum tube amplifiers and clippers. It is also possible to accomplish the same result with a mechanical device known as a chopper.

The chopper is a mechanical switch designed to operate a fixed number of times per second. A cutaway view of a mechanical chopper is shown in figure 5G1. This unit has the contacts arranged for single-pole double-throw switching, center OFF position.

The contact arrangement is shown near the bottom of the drawing. Leads are brought out separately from each of the two fixed contacts and the vibrating reed to pins on the base. These pins are arranged so that the chopper can be plugged into a conventional radio tube socket. In order to reduce operating noise, the entire mechanism is enclosed in a sponge rubber cushion before it is placed in the metal can. By using the chopper in connection with a conventional transformer, amplification can be obtained at the pulse frequency.

Vacuum tubes can be used as electronic choppers. Other amplifiers, known as saturable reactors, are used for a-c motor control. This type of amplifier may sometimes be used in combination with vacuum tubes.

A controller unit in a missile control system responds to an error signal from a sensor. In certain systems an amplifier which is furnishing power to a motor serves as a controller. In this section we will discuss controller units other than amplifiers.

5H2. Types

There are several types of controllers and each type has some feature that makes it better suited for use in a particular missile system than other types.

TRANSFER VALVES. Figure 5H1 shows an application in which two solenoids are used to operate a hydraulic transfer valve. When

RELAYS. Relays are used for remote control of heavy-current circuits. The relay coil may be designed to operate on very small

Figure 5H2 shows a relay designed for controlling heavy load currents. When the coil is energized, the armature is pulled down against the core. This action pulls the moving contact against the stationary contact, and closes the high current circuit. The relay contacts will stay closed as long as the

Figure 5H2.-Low current relay.

Figure 5H3.-Air-actuated relay.

The relay just described has a fixed core. However, some relays resemble a solenoid in that part of the core is a movable plunger. The moving contacts are attached to the plunger, but are electrically insulated from it.

Figure 5H3 shows a form of relay that can be used in a penumatic control system. Two air pressure lines are connected to the air input ports. The relay operates when its arm is displaced by air pressure. A modified design of this type relay might be used in a hydraulic-electric system in which case the diaphragm would be moved by hydraulic fluid pressure.

AMPLIDYNE. An amplidyne can be used as a combined amplifier and controller, since a small amount of power applied to its input terminals controls many times that amount of power at the output. Figure 5H4 shows an amplidyne.

The generator is driven continuously, at a constant speed, by the amplidyne drive motor. The generator has two control field windings that may be separately excited from an external source. When neither field winding is excited, there is no output from the generator, even though it is running. It follows that no voltage is then applied to the armature of the load driving motor. (The field winding of the motor is constantly excited by a d-c voltage.)

The control field windings of the generator are arranged so that the polarity of the excitation voltage from the sensor will determine the polarity of the generator output voltage. The generator output is connected to the load driving motor armature through the latter's commutator. Since the field of the motor is constantly excited by a fixed polarity, the polarity of the voltage applied to the armature will determine the direction of armature rotation.

In a missile control system, any error detected by a sensor must be converted into mechanical motion to operate the appropriate control device. The device that accomplishes this energy transformation is the actuator unit.

The actuator for a specific control system must be selected according to the characteristics of the system. The actuator must have a rapid response characteristic, with a minimum time lag between detection of the error and movement of the flight control surfaces. At the same time, the actuator must produce an output proportional to the error signal, and powerful enough to handle the load.

5I2. Principal types

Actuating units use one or more of three energy transfer methods: hydraulic,

5I3. Hydraulic actuators

Pascal's Law states that whenever a pressure is applied to a confined liquid, that pressure is transferred undiminished in all directions throughout the liquid, regardless of the shape of the confining system.

This principle has been used for years in such familiar applications as hydraulic door stops, hydraulic lifts at automobile service stations, hydraulic brakes, and automatic transmissions.

Generally, hydraulic transfer units are quite simple in design and construction. One advantage of a hydraulic system is that it eliminates complex gear, lever, and pulley arrangements. Also, the reaction time of a

Figure 511 shows that equal input piston displacement will produce the same output piston movement. Actually, this statement is not wholly true because of slight friction and compressibility losses. But for all practical purposes, the motion can be considered to be the same for equal piston displacements.

A different condition is shown in figure 512, where the output piston force has been increased. Because we can't get something for nothing, increased force results in a decrease in piston travel. Keep in mind also, that the output piston could be used as the input piston, and vice versa. Hypothetically then, a small

In the practical application of hydraulics, something must be done to keep fluid in the proper lines. To accomplish this a circulating system is used. This requires pressure, which is furnished by a pump.

PUMPS. The pump used in a hydraulic system must be driven by some power source, usually an electric motor, within the missile. Pumps used in missile systems generally fall into two categories-gear and piston.

A gear type pump is shown in figure 513. It consists of two tightly meshed gears enclosed in a housing. The clearance between the gear teeth and the housing is very small.

As the gears turn past the intake port, fluid is trapped between the gear teeth and the housing. This trapped fluid is carried around the housing to the output port. Because the fluid then has no place else to go, it is forced into the high-pressure delivery line.

A double-action piston pump is shown in figure 514. This arrangement is called double action because fluid is pumped from the reservoir as the piston moves in either direction. To see how this happens, assume that the piston is at the extreme right of the cylinder, and that it has started to move to the left. A slight vacuum will be created as the piston moves, and this will reduce the pressure on valve No. 1. At the same time, system pressure will force valve No.4 shut. Atmospheric pressure, which is admitted to the reservoir through regular inlets, will then act on the fluid which opens valve No. 1. At the same time, fluid to the left of the piston is being compressed. This forces valve No. 2 to close and block the path to the reservoir. The pump pressure forces valve No. 3 to open and, as a result, the fluid on the left side of the piston is forced into the

When the piston reaches the left side of the cylinder, it reverses direction. This creates pressure against valves Nos. 1 and 4 so that valve 1 closes and valve 4 opens. At the same time, valve 3 closes and valve 2 opens.

RESERVOIR. The reservoir shown in figure 514 is a storage compartment for hydraulic fluid. Fluid is removed from the reservoir by the pump, and forced through the hydraulic system under pump pressure. After the fluid has done its work, it is returned to the reservoir to be used again. The reservoir is actually an open tank because of the atmospheric pressure inlets.

VALVES. The valves in the illustrated piston pump are of the flap type, which operate with very small changes in pressure. Another type of valve used in hydraulic systems is the pressure relief valve. As its name implies, it is used to prevent damage to the system by high pressures. Some combination systems use hydraulic pressure regulating switches instead of pressure relief valves.

A typical hydraulic relief valve is shown in figure 515. It consists of a metal housing with two ports. One port is connected to the hydraulic pressure line and the other to the reservoir return line. The valve consists of a metal ball seated in a restricted section of

Should the system pressure become greater than the spring pressure, the ball will be forced away from the opening, and fluid will

ACCUMULATOR. We have shown that hydraulic fluid is stored in a reservoir under open tank conditions. When it becomes necessary to store hydraulic fluid under pressure,

Figure 5I6.-Floating-piston hydraulic accumulator.

a storage space called a hydraulic accumulator is used. The accumulator also serves to smooth out the pressure surges from a double-action pump, which would otherwise cause unsteady operation of control devices to which the actuators are connected.

Figure 5I6 shows a floating-piston hydraulic accumulator. It consists of a closed metal cylinder separated into two compartments by

In operation, the air chamber is charged, through the bottom fitting, with compressed air until the chamber pressure equals the line pressure desired in the hydraulic system. This pressure forces the piston up toward the top of the cylinder. The cylinder is connected to the hydraulic pressure line through the fitting at the top. If the line pressure becomes greater than the air pressure, fluid is forced into the accumulator, and forces the piston down against the air pressure. Should line pressure drop, the air pressure forces the piston up and puts fluid back in the line. This action smooths out variations in pressure during periods of heavy loading, or when the pressure pump lags.

Another type of hydraulic accumulator, the diaphragm type, is shown in figure 5I7. It is built in the form of two hemispherical chambers, which are separated by a flexible diaphragm. Air pressure forces the diaphragm upward. If the system pressure becomes greater than the air pressure, fluid is forced into the fluid chamber. When the system pressure drops, air pressure in the accumulator forces the working fluid back into the system. The diaphragm-type accumulator serves the same purpose in a hydraulic control system as the piston type.

ACTUATOR. The purpose of a hydraulic actuator in a missile control system is to convert fluid pressure into mechanical force great enough to move a control device.

A basic actuator consists of a cylinder with fluid intake and exhaust ports, and a piston which is mechanically connected to the

5I4. Pneumatic

The principal difference between a hydraulic system and a pneumatic system is the use of air rather than hydraulic fluid, as the working medium.

In a pneumatic system, air from a pressure tank passes through delivery tubes, valves, and pressure regulators to operate mechanical units. After the air has done its work, it is exhausted to the atmosphere. It cannot be returned to the tank for reuse. Consequently, air must be stored at a much higher pressure than is necessary for operating the loads in order to have enough pressure to operate the controls as the air supply in the tank diminishes.

A pneumatic control system is shown in figure 5A8, and a brief explanation of the operating sequence is given in the accompanying text.

Generally, motors are used as actuators in electrical control system. The size of the load, and the speed with which it must be moved, determine the type of motor to be used. D-c motors are most often used for driving the heavy loads encountered in missile systems, because d-c motors develop a higher stall torque than do a-c motors. In addition, it is much easier to vary the speed of a d-c motor.

Electrical systems were described in section A, and an electrical pitch control system is shown in figure 5A12.

5I6. Mechanical linkage

We have discussed the various control systems, but have not discussed in detail the mechanical means of linking the flight control surfaces to the actuator. In addition to providing a coupling means, the linkage may also be used to amplify either the force applied or the speed of movement.

A mechanical linkage between an actuator and a load is shown in figure 5I8. The distance,

5I7. Combination systems

Often a number of mechanical systems will be grouped together to form a combination system, as shown in figure 519. This system uses levers, cables, pulleys, and a hydraulic actuator. However, a system using this kind of control is not suited for high speed missiles.

5I8. Followup units

The followup unit in a missile control system plays an important part in obtaining a smooth trajectory with minimum oscillation. It does this by providing continuous information on flight control surface position in relation to the missile axes. The followup signal indicates how the output section of the control system is following the correction data initiated by the sensor unit.

In operation, the followup signal combines with the error signal so as to oppose it. But, the error signal is the larger of the two, and is strong enough to produce the necessary control surface deflection. The error signal amplitude decreases as the missile approaches the correct flight path. When the amplitudes of the error and followup signals are equal, there is no further deflection of the flight control surfaces, because the sum of the two signals is zero.

An electrical followup system is shown in figure 5110. In this system, the error signal is supplied to an electronic mixer where it is combined with the smaller signal from the followup generator. The difference, or resultant, of these signals is fed through an amplifier and controller to the actuator section that operates the control surface. A portion of this signal is also fed to the followup generator, so that the followup signal is proportional to the flight surface deviation from the axis line.

To see how this system operates, assume that the signal from a pneumatic pickoff moves the air relay diaphragm up. The follow-up arm will then move clockwise. This movement causes the valve spool of the air valve to move upward. The valve action admits high-pressure air to the relay, and the pressure forces the piston of the pneumatic actuator to the left. When this happens, the followup

A large signal will create a larger flight control surface deflection before the feedback force becomes great enough to return the followup arm to zero. The spring will then push the followup arm and the air valve in the opposite direction, to move the flight control surface back. Therefore, the spring acts to limit flight surface deflection to a value determined by the error signal, and to return the flight control surface to a position parallel with the missile axis.

A. Introduction
 

From an offensive viewpoint, a missile can be considered as a long-range artillery projectile. In its basic form, a missile is a self-propelled, high-explosive weapon. Like a projectile, it must be carefully aimed in order to follow a trajectory that will take it to the target.

Missiles like projectiles, are influenced by natural forces such as cross winds. But, because a missile usually makes a much longer flight than a projectile, it is subjected to these natural influences for a longer time.

A missile is a very expensive piece of ordnance. If the conventional artillery practice of firing several ranging shots were followed, the cost would be prohibitive. And unguided missiles would be of little value against moving targets, which can take evasive action.

All of these factors add up to this: an accurate means of guiding a missile to its target is an absolute necessity.

Modern guidance systems are far advanced. But progress in electronic and allied equipments is rapid, and our present guidance systems may be out of date within a few years. The ultimate system may be a composite formed from the systems you will read about in this and the following chapters, or it may be entirely different from any of them.

6A2. Definitions

A GUIDED MISSILE may be defined as an unmanned projectile that carries its own flight control equipment. In addition, the missile carries a payload-either of explosives or of scientific equipment. High-altitude missiles are used to obtain data on conditions existing far above the earth. Missiles equipped with guidance control and scientific instruments have been shot into the "mushroom cloud" that forms after an atomic blast, to obtain data on radioactivity, temperatures, and other effects. Since missiles have a number of peacetime applications, they should not be considered exclusively as weapons.

6A3. Purpose and function

The purpose of a guidance system is to control the path of the missile while it is in flight. This makes it possible for personnel at ground or mobile launching sites to hit a desired target, regardless of whether that target is fixed or moving, and regardless of whether or not it takes deliberate evasive action. The guidance function may be based on information provided by sources inside the missile, or on information sent from fixed or mobile control points, or both.

6A4. Basic principles

The guidance system in a missile can be compared to the human pilot of an airplane. One guidance system uses an optical device that guides the missile to the target in much the same way as a pilot, using landmarks for bearings, guides a plane to a landing field.

If landmarks are obscured, the pilot must use another system of guidance. He could, for example, use radio beams. One missile guidance system uses radio or radar beams for guidance; another uses radio to send information to the missile, just as a ground control station might send instructions to a pilot.

We have mentioned radio and radar as primary guidance controls, but these are not the only methods by which a missile trajectory can be controlled. Heat, light, television, the earth's magnetic field, and loran have all been found suitable for specific guidance purposes. Information on all these systems will be given in later chapters.

When an electromagnetic source, such as radio or radar, is used to guide the missile, an antenna and receiver are installed in the missile to form what is known as a SENSOR. The sensor section picks up, or senses, the guidance instructions. Missiles that are guided

For purposes of explanation, missile guidance may be divided into three separate phases. The first is known as the LAUNCHING, or INITIAL phase. The second is called the MIDCOURSE phase, and the last is called the TERMINAL phase. These names refer to different parts of the missile flight path.

6B2. Initial phase

Missiles may be launched from a point at some distance from the guidance equipment. Because the missile does not have aerodynamic stability when it is first launched, the flight controls are locked in the neutral position, and remain locked for a short time after launching. As soon as the controls are unlocked, and the guidance system assumes control, the initial phase of guidance is completed.

6B3. Midcourse phase

The second, or midcourse phase of guidance is often the longest in both distance and

6B4. Terminal phase

The terminal phase is of great importance because it can mean a hit or a miss. The last phase of surface-to-air missile guidance must have high accuracy as well as fast response to guidance signals.

Near the end of the flight, the missile may lack the power necessary to make the sharp turns that are required to overtake and score a hit on a fast-moving target. In order to decrease the possibility of misses, special systems are used. These systems will be described in the following chapters.

A missile guidance system involves a means of determining the position of the missile in relation to known points. The system may obtain the required information from the missile itself; it may use information transmitted from the launching station or other control point; or it may obtain information from the target itself. The guidance system must be stable, accurate, and reliable.

In order to achieve these basic requirements, the guidance system must contain components that will pick up guidance information from some source, convert the information into usable form, and activate a control sequence that will move the flight control surfaces on the missile.

6C2. Sensor

In some respects, the sensor unit is the most important section of the guidance system because it detects the form of energy being used to guide the missile. If the sensor unit fails, there can be no guidance.

Acoustic sensors called hydrophones are often used in torpedo guidance systems. Essentially, a hydrophone is a microphone that works underwater. It picks up the vibrations of ships' propellers, and the torpedo can "home" on this noise. Acoustic sensors are not well suited for airborne missiles.

Heat, or infrared sensors use an active element called a THERMOCOUPLE, or an element known as a BOLOMETER. Either sensor may be used with a lens and reflector system.

Sensors that respond to light use an active element called a PHOTOELECTRIC cell. Another light-sensing system uses a television camera to pick up information and send it back to the launching site by means of a TV transmitter in the missile.

Electromagnetic sensors use radio or radar antennas as active elements. The phase of guidance determines the location of the antenna in the missile structure. For initial and midcourse guidance the antenna is normally streamlined into the tail of the missile. For final phase guidance the sensor may be located in the nose of the missile.

All of these sensors have advantages and disadvantages. Some are well suited for final phase applications and totally unsuited for initial and midcourse guidance. The advantages and disadvantages of each will be covered in later chapters.

6C3. Reference units

The signals picked up by the sensor must be compared with known physical references such as voltage, time, space, gravity, the earth's magnetic field, barometric pressure, and the position of the missile frame. The sensor signal and the reference signal are compared by a computer, which will generate an error signal if a course correction is necessary. The error signal then operates the missile control system.

Gyroscopes are used for space reference. A reference plan is established in space, and the gyro senses any change from that reference.

The earth's gravity can be used as a reference; a pendulum can sense the direction of the

An instrument called a FLUX VALVE, has the ability to sense the earth's magnetic field, and can be used for guidance. The primary purpose of this device is to keep a directional gyro on a given magnetic heading. A gyro operated in this manner may be used to govern the yaw controls of a missile.

Barometric pressure can be used to determine altitude. A guided missile that is set to travel at a predetermined altitude may use an altimeter to sense barometric pressure. Should the missile deviate from the desired altitude, an error signal will be generated and fed to the control section.

Another pressure-type sensor is used to determine airspeed. It compares static barometric air pressure with ram air pressure. The difference between these two pressures provides an air speed indication.

The axis of the missile frame is used as a reference to measure the displacement of the missile control surfaces. (The movement of the control surfaces cannot be referenced to the vertical, or to a given heading, because the reference would change when the missile position changes.)

Selsyns may be used to indicate the angular position of the flight control surfaces with respect to the missile axis. It is also possible to use potentiometers (variable resistors) for this purpose. When this method is used, the potentiometer is fastened to the missile frame and the potentiometer wiper-arm shaft is moved by the control surface.

6C4. Amplifiers

Each of the sensor units we have discussed produces an output; in most cases this output is a voltage. A computer is used to compare the sensor output voltage with the reference voltage. If the missile is off course, the two voltages will not be the same. The computer will then generate an error signal, which will be used to operate the missile control surfaces and bring the missile back on course.

But the computer output power is usually too small to do the actual work of moving the control surfaces. And the output of a sensor